1. Field of the Invention
The present invention relates to a liquid crystal film, a method for manufacturing the same, a polarizing plate, and a liquid crystal display.
2. Discussion of Related Art
There is a continuously increasing demand for a thinner, lighter and wider liquid crystal display (LCD), plasma display panel (PDP) and the like. Also, it is required to improve screen uniformities, contrast ratios, and viewing angles in order to realize a high quality image.
In a display, an optical film, such as a retardation film or a viewing angle-compensation film, may be used to decrease changes in color, to secure a viewing angle, and to improve brightness.
A liquid crystal film using optical anisotropy of a liquid crystal film manufactured by curing polymerizable liquid crystal compounds is known as an optical film.
Liquid crystal molecules may be divided into rod-like liquid crystals and disc-like liquid crystals according to their shapes. There are various alignments of the rod-like liquid crystals, which include homogeneous, homeotropic, tilted, splay, and cholesteric alignments, and the like. Accordingly, the liquid crystal film may have various optical properties that cannot be obtained from an oriented film. For example, a wider variety of physical properties can also be secured by coating polymerizable liquid crystal compounds on the oriented film and then realizing various alignments of the liquid crystal compounds.
The liquid crystal film may be manufactured by forming an alignment layer by applying an alignment agent, such as polyimide or poly(vinyl alcohol), on a substrate; giving an aligning property by rubbing the alignment layer in a predetermined direction; and applying and then aligning polymerizable liquid crystal compounds. However, since the rubbed alignment layer is deficient in an adhesive strength to a liquid crystal layer, there are problems in that the liquid crystal layer is delaminated or contracted under the severe environment, such as a high temperature or high humidity environment. There are also problems in a rubbing process, which are static electricity or scratching which may easily occur due to friction according to a rubbing method and fine dust generated from a rubbing cloth.
In order to solve the problems caused by the rubbing method, a non-contacting alignment method is known, and a photo-aligning method using light irradiations is known as an example thereof. Photo-aligning methods include a method of using a photodimerization reaction of a cinnamate moiety, a coumarin moiety, a chalcone moiety, and the like, a method of using a photoisomerization reaction of a polymer including an azobenzene moiety, a method of using a photodissociation reaction of a polyimide polymer, and the like.
However, according to the above methods, there are still problems in that the alignment layer may have poor heat-stability or photo-stability and contamination may be caused by decomposition products or un-reactive materials.
In addition, the alignment layer is generally formed on a plastic substrate in order to manufacture a retardation film, a viewing angle compensation film or a brightness enhancement film using the polymerizable liquid crystal compounds. However, there is a problem in that a type of a usable substrate is limited in the photo-aligning method. For example, a photo-alignment layer is manufactured by forming an alignment layer by applying a coating solution, which is prepared by dissolving a photo-alignable polymer in a solvent, to a substrate, and then performing aligning through irradiation of light. However, since it is common for a plastic substrate to be easily corroded by a solvent, the substrate has a high probability of incurring damage in a process of manufacturing an alignment layer. When the plastic substrate is damaged in the process of manufacturing the photo-alignment layer, entire physical properties of the film may be deteriorated.
In addition, the plastic substrate may be swelled by a solvent in the process of manufacturing the alignment layer and thus the swelling phenomenon may also decrease the physical properties of the film. For example, when the substrate may be swelled in the process of manufacturing the alignment layer, there may be a lack of cross-linking of the alignment layer according to a level of UV irradiation. The lack of the cross-linking of the alignment layer as described above may cause dissolving and/or swelling again which may be induced by a solvent used to coat a liquid crystal in order to form the liquid crystal layer and thus an orientational property of materials in the alignment layer may be affected. As a result, the aligning property of the liquid crystal may be decreased.
In addition, LCDs are classified into various modes, including twisted nematic (TN), super twisted nematic (STN), vertical alignment (VA), and in-plane switching (IPS) modes according to the ordering of liquid crystal molecules in a liquid crystal panel. Each liquid crystal panel has an intrinsic ordering of liquid crystals and thus there are optical anisotropy differences between the liquid crystal panels.
Accordingly, to compensate the optical anisotropy of the liquid crystal panel, a film having optimized optical physical properties needs to be developed according to a type of the liquid crystal panel.